1. Field of the Invention
The present invention relates to a display device including a light emitting element in which fluorescence or phosphorescence is obtained, and more particularly to a display device in which an active element such as an insulated gate transistor or a thin film transistor and a light emitting element connected therewith are provided to each pixel. Of course, it is possible to apply the present invention in a liquid crystal display device.
2. Description of the Related Art
According to an active matrix display device, a plurality of pixels are arranged in matrix, and a light intensity of each of the pixel based on a video signal is controlled by an active element provided to each of the pixels, thereby displaying a video image. A typical example of the active element is a thin film transistor (hereinafter referred to as a TFT). Currently, a structure using a liquid crystal which responds to a voltage is widely used as means for controlling a light intensity by a voltage written into each of the pixels. In addition, a method of providing a light emitting element to each of the pixels and controlling a light intensity by a current flowing into the light emitting element to display a video image is also known as another structure.
In recent years, one of the most significant light emitting elements is one to which is an organic compound is applied as a phosphor, and also called an organic electroluminescence element because electroluminescence is utilized. Various organic compound materials having light emitting property are known. It is also known that the light emitting mechanism includes fluorescence in which light is emitted through a singlet excitation state and phosphorescence in which light is emitted through a triplet excitation state.
FIG. 12 shows an example indicating a structure of a pixel in a display device. Such a structure of the pixel is disclosed in JP 08-234683 or the like. The pixel shown in FIG. 12 is composed of TFTs 50 and 51, a storage capacitor 52, and a light emitting element 53. With respect to the TFT 50, the gate is connected with a scan line 55, the source is connected with a signal line 54, and the drain is connected with the gate of the TFT 51. With respect to the TFT 51, the source is connected with a power source line 56 and the drain is connected with one terminal of the light emitting element 53. The other terminal of the light emitting element 53 is connected with a power source 57. The storage capacitor 52 is provided so as to keep a voltage between the gate and the source of the TFT 51.
With respect to the operation of the pixel, when the TFT 50 is turned on by a voltage of the scan line 55, a video signal inputted to the signal line 54 is applied to the gate of the TFT 51. When the video signal is inputted, a gate voltage (a voltage difference between the gate and the source) of the TFT 51 is determined according to the voltage of the inputted video signal. Then, a drain current of the TFT 51 which is made to flow by the gate voltage is supplied to the light emitting element 53. The light emitting element 53 emits light according to a value of the supplied current. Light emission by the light emitting element is maintained until a next video signal is inputted. Thus, when such operation is conducted over all pixels in every specific period, a still image and a moving image can be displayed. In addition, when materials for emitting light of each color, red, green, and blue, are applied to phosphors of the light emitting element and the pixels using these materials are arranged, color display can be conducted.
Also, a structure in which TFTs are connected in parallel with an electroluminescence element is disclosed in JP 2000-221903. According to the publication, a plurality of TFTs are connected in parallel with the electroluminescence element. Thus, even if there is a variation in characteristics of TFTs, a variation in light emitting brightness is not caused and uniform display can be obtained.
In order to flow a current into a light emitting element and drive it, a high current drive capacity is required for a TFT. To satisfy this, it is considered to be desirable that an active region of the TFT is made from a polycrystalline silicon film. As a method of forming a polycrystalline silicon film on an insulating surface, there is a method of depositing an amorphous silicon film by plasma CVD method or the like and crystallizing it by a laser beam irradiation. This is called laser annealing. Characteristics thereof include that, even if a glass substrate having a low heat resistance in which a distortion point is 700° C. or lower is used, only a silicon film can be selectively heated to be crystallized without heating the substrate to a large extent. As a laser, a gas laser such as an excimer laser or a solid laser such as a YAG laser and a YVO4 laser is used.
However, with respect to the polycrystalline silicon film produced by the laser annealing, there are variations in size and orientation of a crystal grain. In addition to this, sufficient crystallization is not conducted so that there is a region in which crystallinity is insufficient. When a size of a channel formation region is decreased or a crystal grain is enlarged so that the number of grain boundaries present in a channel is reduced, there is a problem in that a variation in characteristics of TFTs conversely becomes larger. With respect to the pixel shown in FIG. 12, when characteristics of the TFT 51 such as a threshold voltage and an on current are varied for every pixel, even if video signals have the same potential, an amount of a drain current of the TFT 51 varies between pixels. Thus, a variation in brightness of the light emitting element 53 is caused.
Specifically, when display with 64-gray scales is conducted, it is required that a variation of an on current value of the TFT in a saturation region is 1.5% or less. In addition, in order to hold a charge in a storage capacitor portion, it is required that an off current value is 1 pA or less.
However, a first factor with a variation of characteristics of TFTs is laser annealing. Even when the TFTs are arranged in parallel as in JP 2000-221903 and variation in crystallinity of a semiconductor region (channel formation region and regions in which source and drain regions are formed) produced at a size smaller than a width of a laser beam can be suppressed between adjacent TFTs, a variation in brightness which is periodically caused by scanning a linear pulse oscillating laser beam over the entire surface of a pixel region cannot be reduced. This is also due to a variation in characteristics of TFTs. When the periodic characteristic variation is estimated by light emitting brightness of the light emitting element, it is 10% or more. Thus, this cannot be compensated by the parallel connection of TFTs in a pixel.